The long-range goal of the program is to gain a thorough understanding of the complex sensory and neural bases of hearing and hearing impairment. The present proposal addresses a complementary set of questions related to the major theme of the program: spatial localization of complex sound. These questions are addressed through an integrated series of five studies to be conducted by a multi-disciplinary team of established investigators who, collectively, bring to the program a record of productive interaction and expertise in neurophysiology, neuroanatomy, electronic and computer engineering, behavioral testing and human psychoacoustics. Specifically, the collaborative effort includes studies of 1) the peripheral auditory system, from the external ear to the auditory nerve, with emphasis on theoretical models that are experimentally verified, 2) transformations of complex-sound information in the cochlear nuclear complex, with emphasis on speech processing and structure/function relations, 3) binaural processing of complex sound at the level of the midbrain, with special emphasis on mechanisms that underlie the precedence effect and directional hearing as studied with simulated free-field sound, 4) directional selectivity at the level of auditory cortex, examining the primary field and several surrounding cortical areas using simulated free-field sound and 5) sound localization by human listeners, with emphasis on a listener's knowledge or expectations of sound parameters, on sound motion detection, on segregation of multiple sources and on a neural-net model. The work employs a wide variety of methods, including acoustical measurement, signal processing and simulation, theoretical modeling, single-cell recording, animal behavior, neural track tracing, light- and electron microscopy and human psychophysics. The results of this work will provide an increased understanding of how the normal auditory system encodes information pertaining to complex sounds in space. This work is a necessary step in understanding the pathophysiology of the auditory system and will help give insight into strategies for diagnosis, intervention and treatment of hearing disorders and for developing technically sophisticated aids for the hearing impaired, including hearing aids and cochlear prostheses.